The present invention concerns a method and a device for increasing the drying capacity of a pulp or paper machine.
In a pulp or paper machine liquid pulp, so-called stock, is supplied to a head box which distributes the pulp in a thin layer onto a wire. The wire and its associated rolls and suction boxes are called a wire section. In the wire section, the pulp is drained and a web of pulp forms. After the wire section, the web passes a so-called press section, in which more water is removed by pressing. Subsequently, the web is passed on to a drying section.
In the drying section, the web is passed over a number of drying cylinders which are heated by means of steam. The web is usually held against the drying cylinders by means of dryer wires. The drying section is usually divided into a number of drying groups, where each drying group is made up of a number of drying cylinders and a wire. Another way of dividing the drying section is to group the drying cylinders in steam groups, in which each steam group is supplied with steam from the same pipe. The drying groups may coincide with the steam groups.
Steam is supplied to the papermaking machine from a principal steam conduit under constant pressure. It should be noted that all indications of pressure hereinafter relate to overpressure, unless otherwise stated. In most cases this steam, which originates from a steam boiler, has passed one or more turbines, which have converted some of the energy content of the steam into electric power. The steam produced by the boiler usually has a pressure of about 3.5-6 MPa, but pressures of up to about 10 MPa can be found. This steam is called high-pressure steam. From the turbine, steam having two different pressure levels is often conducted: intermediate-pressure steam with a pressure of about 1.0-1.3 MPa and low-pressure steam with a pressure of about 0.2-0.5 MPa. When making thermomechanical pulp (TMP), steam is circulated from the TMP refiner with a relatively low pressure, in the order of 0.15 MPa. The low-pressure steam is conducted to the steam users in the pulp or paper machine. The steam users can, for instance, be said drying cylinders.
Each drying cylinder has an inlet for steam and one or more discharge pipes (so-called siphons) or discharge shovels arranged in the cylinder. Each of the siphons, which can be stationary or rotating, has an opening located near the internal cylinder surface of the cylinder so that it can take up the condensate formed in the drying cylinder. If the pressure drop over the siphon is sufficient, the condensate will be pressed up through the siphon and discharged into one or more condensate vessels/steam separators. If the siphon is empty, steam flows out of the siphon and entrains newly formed condensate in droplet form, which is removed in the condensate vessel. The steam which is removed from a drying cylinder via the siphon is called residual steam (blow-through steam) and it has a lower pressure than the steam supplied to the drying cylinder due to the pressure drop in the drying cylinder, siphon and condensation vessel.
In some situations, it is desirable for economic reasons to reuse the residual steam with a view to using the heat still contained therein, in which case residual steam is sometimes recirculated to the same or one or several adjacent steam groups. However, more often than not this or these steam groups should be supplied with steam having a higher pressure than the pressure of the residual steam. To achieve this, some kind of compressor is installed, which increases the pressure of the recycled residual steam so that it attains the same pressure as the rest of the steam that is fed to the steam group in question. It is, for instance, possible to use a so-called steam ejector which utilises steam with a higher pressure, normally intermediate-pressure steam, to increase the pressure of the residual steam. If residual steam is to be returned, it is necessary to increase the pressure of the residual steam since steam with a certain pressure cannot flow by itself to a point with a higher pressure. The purpose of this return of residual steam is to reuse the heat which is still present in the residual steam after it has passed a first steam group. The energy loss and the investment cost due to the use of the compressor or the ejector are compared with the value of the residual heat of the residual steam when deciding whether to return the residual steam or not. In the above-described system for returning residual steam, the residual steam constitutes a minor part of the total amount of steam supplied to the steam group.
In the above-described prior-art system, there are occasions when the pulp or paper machine is xe2x80x9cdrying limitedxe2x80x9d, i.e. the capacity of the drying section restricts the speed of the web through the papermaking machine. These situations may, for instance, occur when it is desirable to increase the speed of the machine or obtain higher grammage, if the press dry solids content is reduced or if the output dry solids content of the drying section is to be increased. Often only one or a few steam groups limit the maximum production/drying capacity, e.g. the maximum speed. Which steam group or steam groups limit the speed depends on the reason why the machine is xe2x80x9cdrying limitedxe2x80x9d. A limited drying capacity can be avoided, for instance, by lengthening the drying section or by increasing the energy content of the supplied steam, which is usually performed by increasing the steam pressure.
To lengthen the drying section means a great investment, a great fall in production due to the stoppage during reconstruction and results in a machine having a drying section which in many cases is over-dimensioned. Often this solution is not justifiable from an economic point of view.
It is possible to increase the steam pressure of the system by installing a mechanical compressor which raises the pressure of the steam flow that passes the compressor, by increasing the pressure of the low-pressure steam in the entire system or by using the steam in the intermediate-pressure system for the drying groups. However, these solutions result in great costs. Since different drying groups (one or more) can limit the drying capacity of the machine in different cases, it is necessary to install compressors for each of the drying groups, which leads to great investment costs. Relatively high increases in pressure and large steam flows require large and expensive compressors. To increase the pressure of the low-pressure steam or use the intermediate-pressure steam also leads to great costs. A pressure increase of the low-pressure steam corresponds to a reduction of the power available to the turbine. Since the steam flows in a paper machine are often large, also a marginal pressure increase causes great differences in the electric power generated by the turbine, which in turn leads to great economic loss. To increase the pressure is a more or less permanent measure, whereas the limited drying capacity and thus the need of steam with increased pressure often occurs only in certain situations. In addition, the maximum permissible pressure in existing installations is often not much higher than the pressure in normal operation. Moreover, any pressure increase in the steam distribution system often makes it necessary to replace several components in the steam distribution system. Similar problems are associated with the use of intermediate-pressure steam, as the power available to the turbine decreases since the usable steam flow decreases, and in most cases the steam distribution system for intermediate-pressure steam is not dimensioned for the steam flows that are required if the intermediate-pressure steam is to be used for the drying groups.
Thus, a great drawback of prior-art drying systems in pulp and paper machines is that the known methods of increasing the drying capacity are much too expensive.
One object of the present invention is to define a solution to the above-described problems.
According to a first aspect of the invention, this object is achieved by a method for increasing the drying capacity of a steam group in a drying section of a pulp or paper machine, characterised in that a main steam flow having a first steam pressure is fed to the steam group, that the main steam flow is conducted to at least one thermocompressor, which is driven by motive steam having a second steam pressure, which is higher than the first steam pressure, and that the output steam of said at least one thermocompressor is fed to the steam group, the steam pressure of the steam that is fed to the steam group being higher than the first steam pressure.
According to a second aspect of the invention, the above-mentioned object is achieved by a device for increasing the drying capacity of a steam group in a drying section of a papermaking machine, comprising a main steam pipe for supplying a main steam flow to the steam group, characterised in that the device further comprises at least one thermocompressor having an inlet for steam with a first steam pressure, an inlet for motive steam with a second steam pressure and an outlet for output steam, which outlet is connected to the steam group, a steam pipe for supplying motive steam to the motive steam inlet of said at least one thermocompressor, a branch, between the main steam pipe and said inlet of said at least one thermocompressor for steam with a first steam pressure, for conducting the main steam flow to said at least one thermocompressor.
Additional characteristic features and advantages of the invention are stated in the dependent claims and the description below.
Since the main steam flow is conducted to a first thermocompressor, which is driven by motive steam having a pressure that is higher than the pressure of the main steam flow, the steam group can be fed with steam having a pressure that exceeds the pressure of the main steam flow, which means that the drying capacity of the steam group is increased. As motive steam, use can, for instance, be made of intermediate-pressure steam. Alternatively, the invention can be utilised to lower the pressure of the main steam flow with maintained drying capacity by using a thermocompressor.
A thermocompressor is a type of jet ejector in which relatively high-pressure motive steam imparts some of its energy to a main steam flow of steam with a lower pressure. The thermocompressor is made up of three basic components; a nozzle in which the motive steam is allowed to accelerate to high velocity, resulting in its pressure decreasing to a pressure approximate to that of the main steam flow, a mixing section in which the motive steam joins the main steam flow and entrains the same, and a diffusor in which the jet momentum is converted into an increase in pressure. A thermocompressor only needs a relatively limited amount of motive steam, with a higher pressure, to increase the pressure of the main steam flow. As a result, the loss in electric power available from the turbine will not decrease markedly. Besides, a thermocompressor is much simpler and less expensive than the mechanical compressors needed for the same steam demand.
To achieve the highest possible degree of efficiency and economy, it is suitable to dimension a first thermocompressor for the pressure increase level and the steam capacity mostly required.
When a temporary need for additional pressure rise/steam capacity occurs, it is possible to lead a relatively small amount of the motive steam past the thermocompressor directly to the steam group.
Additional pressure rise/steam capacity can also be provided by conducting some of the main steam flow to a second thermocompressor which is connected in parallel to the first thermocompressor.